1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a technique effective when applied to a liquid crystal display device that includes a light guide plate type backlight unit.
2. Description of the Related Art
Conventional liquid crystal display devices include ones that use a light guide plate type backlight unit. In the light guide plate type backlight units, a light guide plate is disposed in, for example, a place that overlaps with the display area of a liquid crystal display panel, and a light source such as a light emitting diode or a fluorescent light is placed at an end of the light guide plate. Compared to direct lit backlight units having a light source in a place that overlaps with the display area of a liquid crystal display panel, the light guide plate type backlight units require fewer light sources and reduce the power consumption of a liquid crystal display device. Also, the light guide plate type backlight units can be made thin more easily than direct lit backlight units, and are employed by liquid crystal display modules for use in portable electronic devices such as cellular phones. An increasing number of large-sized liquid crystal display devices including liquid crystal television sets have recently used the light guide plate type backlight units as well for the purpose of obtaining a thinner shape and a lighter weight, and reducing power consumption.
In the light guide plate type backlight units, however, the intensity of light (light amount) emitted from a light source attenuates while, for example, traveling through the light guide plate. This often causes the light amount to differ between light that exits the light guide plate of the light guide plate type backlight unit from a place close to an incidence end surface and light that exits the light guide plate from a place far from the incidence end surface, for example. As a result, the surface luminance evenness is lowered in a planar beam of light for irradiating a liquid crystal display panel.
The light guide plate type backlight units therefore enhance the surface luminance evenness of an exiting planar beam of light by using, for example, a light guide plate that grows larger in number of reflection patterns for extracting light or in dimensions as the distance from the incidence end surface increases.
The conventional light guide plate type backlight unit usually includes one light guide plate. Consequently, when a light guide plate type backlight unit is used in a liquid crystal display device that has “hold type” light emission characteristics, a motion blur often occurs due to the luminance response characteristics of respective pixels.
One of methods that have been proposed to remedy the motion blur as described above is a method for providing two light guide plates that have different light reflection patterns and for emitting light intermittently at different timing using a light source for one light guide plate and a light source for another light guide plate emit light (see, for example, JP 2004-286803 A).
In a liquid crystal display devices including a conventional backlight unit, the surface luminance of a planar beam of light for irradiating the display area is generally desired to be even or to be distributed such that the luminance decreases gradually and concentrically from the center toward the perimeter.
However, as mentioned above, a planar beam of light emitted from the conventional light guide plate type backlight unit tends to become lower in luminance as the distance increases from the light incidence side surface of the light guide plate. Realizing an even surface luminance of a planar beam of light is relatively easy in the conventional light guide plate type backlight units, but it is difficult to decrease the luminance gradually and concentrically from the center toward the perimeter.
Some of recent liquid crystal display devices including a backlight unit are desired to two-dimensionally control the surface luminance of a planar beam of light for irradiating the display area by, for example, area-based control called local dimming or such.
The area-based control is a method for, for example, dividing the display area into a plurality of small areas and controlling the luminance of irradiation light for each of the small areas separately. A liquid crystal display device that employs area-based control can improve the contrast of a displayed image by, for example, setting a low luminance to light that irradiates a small area where the display gray level is low in many pixels and by setting a high luminance to light that irradiates a small area where the display gray level is high in many pixels.
In the direct lit backlight units, a plurality of light sources can be arranged into a matrix pattern in a place that overlaps with the display area. It is therefore relatively easy to control the surface luminance of a planar beam of light two-dimensionally as in the area-based control by liquid crystal display devices including the direct lit backlight unit.
In the light guide plate type backlight units, on the other hand, the surface luminance of a planar beam of light may be two-dimensionally controlled by devising a light extracting structure, for example, a placement of a light reflection pattern to be provided in the light guide plate.
However, the surface luminance distribution of a planar beam of light in the conventional light guide plate type backlight units is determined mainly by a light extracting structure such as a light reflection pattern which is provided in the light guide plate. This makes it difficult for liquid crystal display devices that include the conventional light guide plate type backlight unit to switch between setting the surface luminance of the planar beam of light even for one circumstance and varying the surface luminance of the planar beam of light two-dimensionally for another circumstance. Another problem is that dynamically performing two-dimensional control of the surface luminance as in, for example, the area-based control is difficult in liquid crystal display devices that include the conventional light guide plate type backlight unit.